Lanthanide trihalides properties

The lanthanide trihalides have been subject of studies for many decades. The main incentive has been the scientific interest in the physical and chemical properties of compounds of the trivalent lanthanide ions, which are unique in the period system as they regularly vary along... 

These high temperature processes can be modelled adequately by equilibrium thermodynamics. For such calculations reliable thermodynamic data are a priority. Although numerous studies of the thermodynamic properties of the lanthanide trihalides have been published in the past, the available information is still not complete. But because the properties change regularly within the lanthanide series, estimates can help to obtain the data that are lacking. 

In the present chapter we have presented a careful evaluation of the thermodynamic and related properties of the lanthanide trihalides. It is shown that the properties of these compounds vary regularly within the four series (F to I) and in most cases clear trends are observed. 

The properties of the liquid lanthanide trihalides depend strongly on the atomic number of the halide. The variation in the heat capacity of the lanthanide fluorides indicates a strongly ionic behaviour of the melts with a concomittent irregular trend related to the electronic configuration of the lanthanide ions. In the lanthanide chlorides, bromides and iodides the trend becomes systematically more constant, indicating an increasing molecular nature of the melts. 

Although the literature on the thermodynamic and related properties of the lanthanide trihalides on which our evaluation and conclusions are based is extensive, it is far from complete. As we have demonstrated in many instances the gaps in the experimental information can be filled with estimates (e.g., the standard entropies and the enthalpies of formation) based on the observed systematics. However, this is not always possible for the following reasons ... 

This volume of the Handbook illustrates the rich variety of topics covered by rare earth science. Three chapters are devoted to the description of solid state compounds skutteru-dites (Chapter 211), rare earth-antimony systems (Chapter 212), and rare earth-manganese perovskites (Chapter 214). Two other reviews deal with solid state properties one contribution includes information on existing thermodynamic data of lanthanide trihalides (Chapter 213) while the other one describes optical properties of rare earth compounds under pressure (Chapter 217). Finally, two chapters focus on solution chemistry. The state of the art in unraveling solution structure of lanthanide-containing coordination compounds by paramagnetic nuclear magnetic resonance is outlined in Chapter 215. The potential of time-resolved, laser-induced emission spectroscopy for the analysis of lanthanide and actinide solutions is presented and critically discussed in Chapter 216. 

Joubert et al presented a systematic study of the structural and bonding properties of selected lanthanide trihalide molecules, LnXa (Ln = La, Gd, Lu X = F, Cl). An ELF analysis revealed typical ionic bonding properties, emphasizing the increasing ionic character of the Ln-X bonds through the rare-earth series. Moreover the authors pointed out a strong distortion of the outer core shell of the metal. 

The lanthanide and actinide halides remain an exceedingly active area of research since 1980 they have been cited in well over 2500 Chemical Abstracts references, with the majority relating to the lanthanides. Lanthanide and actinide halide chemistry has also been reviewed numerous times. The binary lanthanide chlorides, bromides, and iodides were reviewed in this series (Haschke 1979). In that review, which included trihalides (RX3), tetrahalides (RX4), and reduced halides (RX , n < 3), preparative procedures, structural interrelationships, and thermodynamic properties were discussed. Hydrated halides and mixed metal halides were discussed to a lesser extent. The synthesis of scandium, yttrium and the lanthanide trihalides, RX3, where X = F, Cl, Br, and I, with emphasis on the halide hydrates, solution chemistry, and aspects related to enthalpies of solution, were reviewed by Burgess and Kijowski (1981). The binary lanthanide fluorides and mixed fluoride systems, AF — RF3 and AFj — RF3, where A represents the group 1 and group 2 cations, were reviewed in a subsequent Handbook (Greis and Haschke 1982). That review emphasized the close relationship of the structures of these compounds to that of fluorite. 

The thermochemistry of rare-earth trifluorides was summarized in Gmelin Hand-buch (1976) and the thermochemistry of rare-earth tribromides and triiodides was summarized in Gmelin Handbuch (1978). The thermochemistry of trivalent rare-earth trichlorides was critically assessed by Morss (1976). Enthalpies of formation of most of the lanthanide tribromides were determined by Hurtgen et al. (1980). Thermodynamic properties for europium halides were assessed by Rard (1985). Only enthalpies of formation of Sc, Y, Dy and Tm triiodides have been redetermined since the classical work of Hohmann and Bommer (Morss and Spence 1992). A recent set of literature values of enthalpies of formation of rare-earth solid and gaseous trihalides has been published, accompanied by Born-Haber cycle estimated values for all trihalides (Struck and Baglio 1992). 

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